1. Field of the Invention
The present invention relates generally to the art of making M(PO.sub.3).sub.3 and more particularly to the preparation of the trimetaphosphates of iron and chromium.
2. The Prior Art
U.S. Pat. No. 4,216,190 to Neely teaches a method for preparing predominantly the B-form of aluminum trimetaphosphate by the addition of ammonia to a solution of aluminum dihydrogen phosphate, Al(H.sub.2 PO.sub.4).sub.3. A white precipitate is formed, which is directly converted to B-form of aluminum trimetaphosphate, Al(PO.sub.3).sub.3, by a single elevated temperature treatment.
In a study of the effects of grinding on the structure and properties of various aluminum phosphates. Tsuhako et al report mixing .alpha.-alumina and ammonium dihydrogen phosphate, and heating at 500.degree. to 550.degree. C. for 20 hours to obtain the B-form of Al.sub.4 (P.sub.4 O.sub.12).sub.3.
3. The Present Invention
The present invention provides a simple method for preparing the trimetaphosphates of trivalent species such as iron and chromium. A phosphate compound such as ammonium phosphate is mixed with a compound which comprises an oxygen-containing anion and a cation of an element that has a trivalent ionic species. The mixture is subjected to a single treatment at elevated temperatures to yield the trimetaphosphate.
4. Description of the Preferred Embodiment
An oxygen-containing compound comprising an element having cationic species capable of reacting with a phosphate compound to form a trimetaphosphate is mixed with a suitable phosphate compound. Useful elements are those wherein the trivalent cation has an ionic radius of at least 0.5 Angstroms. The anion may be simple oxide or an oxygen-containing species such as hydroxide. Preferred compounds include metal oxides such as iron oxides, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, and chromium oxides, CrO.sub.3 and Cr.sub.2 O.sub.3. The phosphate compound is preferably ammonium phosphate, which may be monobasic or dibasic. Both the metal oxides and the phosphate are preferably in powder form.
The metal oxide and ammonium phosphate are mixed together thoroughly, preferably in approximately stoichiometric quantities. Although the dry powders are preferred, the materials may also be combined in an aqueous slurry or even in solution. However, a major advantage of the present invention is the use of dry powder reactants. The blended reactants are then subjected to a single elevated temperature treatment. Heating at a temperature of about 630.degree. to 640.degree. C. is most preferred, in order to keep the heat treatment time relatively short. With masses of reactants in the ranges illustrated by the following examples, about 25 minutes is generally sufficient for forming the iron trimetaphosphate, while about 45 minutes is typically required for the chromium trimetaphosphate. At a given treatment temperature, larger batches will require longer treatment times. The resultant product may be analyzed by X-ray diffraction to determine the yield of the metal trimetaphosphate.